Kujan Sahani, a worker from eastern Bihar state, complained that his nose and eyes burned and he was feeling breathlessness.

But despite the dangers of a heavily polluted river, some Indians posed happily for photos with the foam.

Environmental activists say many rivers across India have become dirtier as the country’s economy develops, with city sewage, farming pesticides and industrial effluents freely flowing into waterways despite laws against polluting.

And India isn’t the only country to witness the toxic foam phenomenon, with photos showing a river in North Jakarta covered in the substance.

It is unclear what caused the foam to appear in Indonesia.

It comes as air pollution in New Delhi and surrounding areas reached this year’s worst level on November 3 and 4.

Hindu women worship the Sun god in the polluted waters of the river Yamuna during the Hindu religious festival of Chatth Puja in New Delhi, India

Water from the river is chemically treated before being supplied to Delhi’s millions of residents as drinking water

Authorities declared a health emergency in Delhi last week, with air pollution so bad that schools were forced to close

A Supreme Court-appointed panel temporarily banned construction activity in the New Delhi region to control the dust in the air.

Authorities also barred smoke-spewing cargo trucks from city streets, experimented at limiting the number of cars on the road and tried to snuff out stubble and garbage fires and ordered builders to cover construction sites to stop dust from enveloping the area.

But India’s air pollution problem extends far beyond the more than 20 million residents of New Delhi.

Its northern belt is one of the most densely populated parts of the planet.

The industrial hub of Kanpur, home to 3 million people, is followed by 13 Indian cities on a World Health Organization (WHO) list of places with the worst air.

Although air quality readings have consistently stayed above 500 for consecutive days, Kanpur, like most other Indian cities, lacks the infrastructure necessary to fight air pollution, and has adopted few emergency measures.

Environmental activists say many rivers across India have become dirtier as the country’s economy develops

‘The water is absolutely black in color,’ said Hari Lal, a New Delhi resident who was on the river bank Wednesday

A Hindu couple worships the Sun god in the polluted waters of the river Yamuna during the Hindu religious festival of Chatth Puja in New Delhi, India

The shocking toxic foam is believed to have been caused partly by high ammonia levels emanating from industrial pollutants

Air pollution in New Delhi and surrounding areas reached this year’s worst level on November 3 and 4